Storm Forecast
Valid: Mon 30 Apr 2007 06:00 to Tue 01 May 2007 06:00 UTC
Issued: Sun 29 Apr 2007 18:12
Forecaster: TUSCHY

SYNOPSIS

The main change compared to the past few days will be a developing cut-off depression just northwest of the Iberian Peninsula.
Otherwise, strong high pressure area ( centered northeast of Scotland ) will be stationary, assisting in an outbreak of cold air on its downstream side, which affects parts of northeastern Europe.
Weak pressure anomalies over the Mediterranean will be present and diurnal driven thunderstorm activity can be expected in the highlighted areas.

DISCUSSION

...France...

Anticipated thunderstorm activity will be a mixture of again diurnal driven but also increasingly dynamically driven thunderstorm activity.
On the coarse scale, the main feature of interest is a developing upper-level cut-off northwest of Spain, which slowly shifts towards the southeast, reaching Spain during the evening and night hours.
Downstream of this system, GFS indicates numerous weak vorticity maxima, crossing mainly western and northwestern France from SE-NW. In addition, a weak coupled jet configuration is present over the Bay of Biscay and attendant favorable upper-level divergence field should also affect parts of western / northwestern France.
At lower levels, the streamline charts of the moisture advection again have the strongest convergent signals over central and northwestern France in a weakly capped environment. Highest instability values should evolve on the eastern side of the convergence zone, where a somewhat better well mixed boundary layer supports steeper lapse rates and SBCAPE values of up to 1KJ/kg.
Those storms, evolving along this convergence zone over central France will locally produce subsevere hail and gusty winds, but weak shear should preclude storm organisation.

Attention then turns to extreme northwestern France during the night hours.
Arriving cold front from the Bay of Biscay ( 21Z onwards ) will maximize convergence over NW France, while UL divergence will be present, too. A weak vort max. will cross this area from the SE and should contribute to a favorable environment for enhanced thunderstorm activity.
Shear parameters are not that impressive, but should be enough for some storm organisation and a cluster of storms with an isolated large hail risk can be expected.

The issued and well towards the S / SE extended level-1 is only valid during the evening and night hours.

...NW / N Spain...

Diurnal driven thunderstorm activity can be expected over Spain. A combination of only slowly strengthening shear and weakening lapse rates will preclude any severe weather threat.
As the upper-level cut-off draws near Spain, thunderstorm activity should be re-activated mainly over NW and N-Spain. Disjunction of shear and instability over NW Spain and constantly weakening lapse rates over N-Spain will avoid any higher probabilities.

...Central Mediterranean...

Another round of diurnal driven thunderstorms can be expected. The predominantly storm mode will be composed of scattered pulsating storms. An isolated large hail and strong wind gust report can't be excluded mainly with maturing storms.

...Extreme western Russia...

In the range of a low-level depression, some low-end instability evolution looks reasonable mainly along southward advancing cold front and along the prowled occlusion.
While enhanced DLS along the cold front indicates an isolated large hail / severe wind gust risk, significantly enhanced SRH 1km along the occlusion favors in addition an isolated tornado threat.
A level-1 was issued for the latter one because of the low-end tornado chance.